1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or C-MOS sensor used in a compact image pickup device and more particularly to imaging lenses composed of five lenses which are built in image pickup devices mounted in mobile terminals such as mobile phones and smart phones, which are becoming increasingly compact and thin, and PDAs (Personal Digital Assistances), game machines and information terminals such as PCs.
2. Description of the Related Art
In recent years, the market of mobile terminals with an image pickup device has been expanding increasingly. Today, most of such mobile terminals have a camera function. Nowadays, as for the camera performance of these mobile terminals, a high pixel density camera function comparable to a digital still camera is becoming mainstream. Furthermore, for the reasons of convenience and appearance, the demand for thinner models is growing, leading to a stronger need for smaller and thinner built-in image pickup devices. With this background, an imaging lens built in an image pickup device using a high pixel density image sensor is expected to provide higher resolution and be more compact and thinner and also to be a fast lens (namely a lens with a small F-value). At the same time, the imaging lens is strongly anticipated to provide a wide angle of view so that an image of a wide object can be taken.
Generally an imaging lens which copes with the need for compactness, thinness, and high performance is composed of a plurality of lenses. Conventionally, in order to deal with VGA up to 1 megapixel, an imaging lens composed of two or three lenses has been widely used for its advantage in terms of size and cost. Also, many types of imaging lenses composed of four lenses which are intended to cope with the need for an increase in the number of pixels have been proposed. Recently, imaging lenses composed of five lenses which can provide higher performance than those composed of four lenses have also been proposed in order to cope with the need for compactness and increase in the number of pixels as mentioned above. The present invention provides an imaging lens which is composed of five lenses to meet the above needs.
JP-A-2009-294528 (Patent Document 1) discloses an imaging lens composed of five lenses which are arranged in order from the object side as follows: a first lens with positive power having a convex surface on the object side, an aperture stop, a second lens with a meniscus shape near the optical axis, a third lens having a convex surface on the image side near the optical axis, a fourth lens in which both surfaces are aspheric and the peripheral area of the image side surface is convex, and a fifth lens in which both surfaces are aspheric and the peripheral area of the image side surface is convex, and among the second to fifth lenses there is only one negative lens with an Abbe number of 30 or less.
JP-A-2010-026434 (Patent Document 2) discloses an imaging lens composed of five lenses in which a positive first lens, a positive second lens, a negative third lens, a positive fourth lens, and a negative fifth lens are arranged in order from the object side.
The imaging lens described in Patent Document 1 has a ratio of total track length (TTL) to maximum image height (IH) (TTL/2IH) of about 1.0, so it is relatively compact. However, since its F-value is about 3.0, therefore, the imaging lens is not considered to provide enough brightness to cope with the increasing trend toward image sensors with a larger number of pixels. Although the imaging lens described in Patent Document 2 is a lens system which has an F-value of 2.05 to 2.5 to ensure sufficient brightness and provides high aberration correction capability, the power of its first lens is weak, so it is disadvantageous in order to achieve thinness.